The invention relates to an auto-focus camera, and more particularly, to such camera in which an offset from as predetermined focussed position where an object being photographed is to be imaged is detected and used to drive a taking lens to an in-focus position.
As is well recognized, a focus control employed in auto-focus cameras includes two techniques, one feeding the entire taking lens and the other feeding only part of the taking lens. FIG. 15 illustrates an optical path diagrammatically which is followed when feeding the entire taking lens. In this Figure, an imaging plane for an object D will be in focus or coincides with a film surface .circle.f when a taking lens is located at .circle.d , and another object E will be in focus when the taking lens is located at .circle.e . Denoting the focal length of the taking lens by f, a spacing between the principal points by .DELTA., a spacing between the object D and the lens position .circle.d by (f+x.sub.D), a spacing between the position .circle.d and the film surface .circle.f by (f+x'.sub.D), a spacing between the object E and the lens position .circle.e by (f+x.sub.E), and a spacing between the lens position .circle.e and the film surface .circle.f by (f+x'.sub.E), it follows that EQU x.sub.E x'.sub.E =f.sup.2 EQU or EQU x'.sub.E =f.sup.2 /x.sub.E ( 1)
When the taking lens moves from the position .circle.e to the position .circle.d , the object E can no longer be imaged upon the film surface .circle.f , but will be imaged on a plane .circle.c which is spaced from the film surface .circle.f by a distance t.sub.DE. Representing a spacing between the object E and the lens position .circle.d by (f+x.sub.DE) and a spacing between the lens position .circle.e and the imaging plane .circle.c by (f+x'.sub.DE), it will be seen from the illustration that EQU f+x'.sub.DE =f+x'.sub.D +t.sub.DE EQU or EQU x'.sub.DE =x'.sub.D +t.sub.DE ( 2)
ti Since EQU x.sub.DE x'.sub.DE =f.sup.2 ps EQU we have EQU x.sub.DE =f.sup.2 /x'.sub.DE ( 3)
In order to bring the imaging plane for the object E into coincidence with the film surface .circle.f or to achieve an in-focus condition, it will be seen that the taking lens may be fed through a distance (x'.sub.E -x'.sub.D).
It will be seen from the illustration that the distance form the object E to the film surface .circle.f is given by the following equation: EQU f+x.sub.DE +.DELTA.+f+x'.sub.D =f+x.sub.E +.DELTA.+f+x'.sub.E EQU Accordingly EQU x.sub.DE +x'.sub.D =x.sub.E +x'.sub.E ( 4)
The substitution of the equation (3) into the equation (4) yields EQU (f.sup.2 /x'.sub.DE)+x'.sub.D =x.sub.E +x'.sub.E ( 5)
The substitution of the equation (2) into the equation (5) yields EQU f.sup.2 /(x'.sub.D +t.sub.DE)=x.sub.E +x'.sub.E -x'.sub.D
Solving this equation for t.sub.DE, we have EQU t.sub.DE =f.sup.2 /(x.sub.E +x'.sub.E -x'.sub.D)-x'.sub.D
Assuming that x'.sub.D and x'.sub.E are sufficiently small as compared to x.sub.E, the above equation can be approximated as follows: EQU t.sub.DE .perspectiveto.(f.sup.2 /x.sub.E)-x'.sub.D ( 6)
The substitution of the equation (1) into the equation (6) yields EQU t.sub.DE .perspectiveto.x'.sub.E -x'.sub.D
This means that the offset t.sub.DE of the imaging plane for the object E is substantially equal to a difference between the amount of travel by which the taking lens is fed in reaching the positions .circle.e and .circle.d . Accordingly, by assuming a conversion coefficient kl which remains invariant with a focussing lens position or an offset in the position of the focus, a quantity l by which the taking lens is to be driven can be expressed with respect to a focal offset t.sub.DE as follows: EQU l=kl.multidot.t.sub.DE
However, if x'.sub.D and x'.sub.E are not negligible in comparison to x.sub.E, or when the travel by which the taking lens is fed increases or where the entire taking lens is not fed but only part of the lens (hereafter referred to as a focus group) is fed for purpose of focussing, there occurs a change in the focal length or spacing between the principal points, whereby the described approximation no longer applies. For this reason, the travel of the focus group is related to an offset of the imaging plane in a non-linear manner, causing the value of the conversion coefficient kl to change depending on the position of the focus group. To accommodate for this, the value of kl is chosen to be small in order to prevent a failure of achieving an in-focus condition as a result of a diversion thereof. Alternatively, for a lens which exhibits an increased magnitude of non-linearity, the position of the focus group is detected, and the kl value which applies only in the vicinity of such position is employed.
This increases an error in the travel by which the focussing lens is to be driven, and also makes it difficult to achieve a movement of the focussing lens into an intended position in one pass, requiring a repeated detection of a focussed position followed by sequentially driving the focussing lens. This resulted in an increased length of time required for the focussing operation, which prevented a rapid photographing operation, and also resulted in the difficulty to achieve a high accuracy of the focussing operation.
To cope with this problem, an auto-focus arrangement is disclosed in Japanese Laid-Open patent application No. 78,519/1987 in which means is provided for calculating a travel by which a focussing lens group is to be fed, by determining a reference coefficient which relates to a focus detecting output signal and a lens movement and deriving a movement coefficient therefrom which may be utilized to drive a focussing lens group. The present applicant has previously proposed an auto-focus camera in which a travel for a focussing lens group is derived by utilizing a conversion coefficient which depends on a lens travel and a focal offset (Japanese pending Patent Application No. 121,790/1987).
However, these proposals fail to derive a movement coefficient in the event a movement of the focussing lens group is non-linear, and if such coefficient is derived, the very use of the movement coefficient requires an increased length of time and labor in reaching a travel of the focussing lens which matches the focal offset in a one-to-one correspondence. Also, the calculation involves a systematic error of an increased magnitude because a number of multiplications and divisions are repeated, eventually resulting in an error in the travel of the focussing lens.
As an overall consequence, where the lens is significantly defocussed, the focus group cannot be driven to its in-focus position in one lens movement, but a plurality of measurements of the distance and associated lens drive have to be repeated to achieve an in-focus condition, thus requiring an increased length of time.
In addition, an offset of the imaging plane from the film surface does not match the amount of defocus, as will be discussed below. FIG. 16 diagrammatically illustrates an optical path of an auto-focus (hereafter abbreviated as "AF") arrangement. In this Figure, an AF focus detecting sensor 40 includes a number of elements which are disposed to view a pupil position of a taking lens 43 through a fry eye lens 41 and a contact lens 42. As a result of using the contact lens 42, an error or offset of the imaging plane for the light from an object being photographed is inevitable. As illustrated in FIG. 17A, if it is forwardly focussed, a true defocus Er1 does not match a defocus Ers1 detected by the sensor 40. Where it is focussed rearwardly as shown in FIG. 17B, a true defocus Er2 again does not match a defocus Ers2 detected by the sensor 40. FIG. 18 graphically shows a hyperbolic relationship between the true defocus Er and the defocus Ers detected by the sensor 40. In this Figure, the relationship shown in phantom line will be reached when Er=Ers.
Representing the focal length of the taking lens 43 by fc and taking the direction in which the light travels as positive, there is the following relationship: EQU 1/fc=-1/Er+1/Ers
Solving this for Er yields
Er=fc.multidot.Ers/(fc-Ers) (7)
Thus, to obtain the true defocus Er, the defocus Ers detected by the sensor 40 and the focal length fc of the contact lens 42 must be substituted into the equation (7) for calculation.
In the prior art practice, a correction according to the equation (7) which is applied to a detected output from the offset sensor is applied within the sensor, and the corrected defocus is employed as an offset between the film surface and the imaging plane or as the focal offset, which has been made the basis for subsequent calculations.
The correction according to the equation (7) has been applied within the sensor of the focal detector in a conventional auto-focus camera, but depending on the technique employed for the calculation, the correction itself resulted in an increased error or the calculation took too long a time in order to improve the accuracy. Since the true defocus as corrected is subsequently converted into a signal relating to the lens drive value within the camera, it will be seen that there may be an unnecessarily iterated calculation process.